Traditionally, the “professional” antigen-presenting cells (APCs) of the myeloid lineage, dendritic cells and macrophages, have been viewed primarily as accessory cells, functioning simply to assist T cell activation. Recently, however, it has become clear that myeloid-lineage APCs exert a profound influence on T cells, regulating both the nature of the response (humoral versus cellular immunity) and, in some cases, even whether a response occurs at all (activation versus anergy). This has been the subject of recent reviews by Fearon and Locksley (Science 1996; 272:50–54) and Trinchieri and Gerosa ( J. Leukocyte Biol. 1996; 59:505–511). Currently, the biology of myeloid-lineage APCs is not well understood. Dendritic cells and macrophages appear to derive from a common progenitor in the myelomonocytic lineage (Szabolcs, et al. Blood 1996; 87:4520–4530), but their markedly different functional characteristics are determined during a complex process of hematopoietic differentiation, which continues well after their exit from the bone marrow (Thomas, et al. Stem Cells 1996; 14:196–206). Hematopoietic differentiation has traditionally fallen outside the purview of classical immunology.
Macrophages enter the tissues at the immature stage of circulating monocytes. Using in vitro models, it has been shown that the cytokine milieu which they encounter at this early stage determines the phenotype which they will subsequently adopt. Under the influence of certain cytokines (in humans, usually GM-CFS plus IL-4 or TNF), monocytes differentiate in vitro into cells which closely resemble dendritic cells (Mackensen, et al. Blood 1995; 86:2699–2707; Rosenzwajg, et al., Blood 1996; 87:535–544). In the presence of inflammatory cytokines they differentiate into macrophages activated for antigen presentation and host defense (Munn, et al., Cancer Res. 1993; 53:260–2613; Morahan, et al. In: Heppner G H, Fulton A M, eds. Macrophages and Cancer. Boca Raton, Fla.; CRC Press, 1988:1–25). In the absence of inflammatory cytokines, monocytes differentiate under the influence of their lineage-specific growth factor, MCSF, into a type of macrophage which inhibits, rather than supports, T cell activation (Munn, et al. J. Immunol. 1996; 156:523–532).
The adaptive immune system must tailor the T cell repertoire so as not to respond to self antigens. The classical model (reviewed by Nossal in Cell 1994; 76:229–239) holds that autoreactive T cell clones are deleted in the thymus via the process of negative selection (encounter with antigen at the immature thymocyte stage triggers apoptosis, resulting in clonal deletion). Although the thymus undoubtedly provides a major site of negative selection, there are two difficulties with this model. First, it would seem unlikely that every developing T cell could be exposed to every self protein during its relatively brief transit through the thymus. Second, autoreactive T cells are empirically found in the peripheral blood of normal, healthy hosts (Steinman Cell 1995; 80:7–10). This suggests that there must exist some additional means of tailoring the T cell repertoire after the T cells have left the thymus, a process now designated peripheral tolerance. Multiple mechanisms have been proposed to contribute to peripheral tolerance (for recent reviews see Steinman L. Cell 1995; 80:7–10; Mondino, et al. Proc. Natl. Acad. Sci USA 1996; 93:2245–2252; and Quill H. J. Immunol. 1996; 156:1325–1327).
Most recent studies support a model in which dendritic cells are the primary physiologic route of antigen presentation to T cells (Thomas et al. 1996). Under normal circumstances, this process is felt to occur only in lymph nodes. Given this exclusive role for dendritic cells in initiating immune responses, tissue macrophages represent a paradox. They are professional APCs, but they are also professional scavengers of all manner of damaged cells and proteins, and hence take up a huge array of self antigens. Moreover, unlike dendritic cells, many of them constitutively express MHC and costimulatory ligands (Azuma, et al. Nature 1993; 366:76–79) and function as APCs in vitro (Unanue, et al. Science 1987; 236:551–557), implying they are constantly prepared to present antigen.
It is not known how they avoid provoking autoimmune responses. One possibility is that T cells never encounter tissue macrophages. This may indeed be the case for naive T cells, since they are not thought to circulate through tissues (Springer, et al. Cell 1994; 76:301–314). However, at times of injury and inflammation, many self antigens unavoidably enter the normal antigen-presentation pathway along with legitimate foreign antigens (either because the dendritic cell has no way to discriminate between the two, or due to influx of debris from damaged tissues into the draining lymph nodes (Steinman 1995)).
Certain pathological conditions, such as AIDs (caused by the human immunodeficiency virus, HIV) and latent cytomegaloviral (CMV) infections, are extremely difficult to treat since the macrophages act as reservoirs for the viruses. Even though the cells are infected with virus, they are not recognized as foreign. It is not known why these cells are protected from the host's immune system.
It is therefore an object of the present invention to identify mechanisms by which tissue macrophages regulate T cell activation in order to modulate autoimmune responses to the self-derived antigens which they present, especially in the context of infections with facultative intracellular pathogens, such as HIV and CMV.
It has long been a mystery why a pregnant individual does not reject her fetus as foreign. Many theories have been proposed, and various mechanisms suggested. Being able to understand and control this phenomena would be of benefit both for the development of contraceptives or aborticides, as well as in treatment of some women who are unable to carry a fetus full term. Medawar, 1953, Symp. Soc. Exp. Biol. 7, 320–338, pointed out 45 years ago that the mammalian conceptus ought to survive gestation because it carries and expresses paternally-inherited polymorphic genes that provide maternal immune responses when expressed by other tissues. The paradox presented by survival of fetal allografts has not yet been explained in mechanistic terms despite much research on the immunology of mammalian reproduction.
Three factors that might explain the immunological paradox of fetal survival are: (1) anatomic separation of mother and fetus, (2) antigenic immaturity of the fetus and (3) immunologic “inertness” (tolerance) of the mother (Medawar 1953). Recently, attention has focused on the third possibility based on evidence that the entire maternal T cell repertoire is transiently tolerized to paternal MHC class I alloantigens during pregnancy (Tafuri, et al., 1995, Science 270, 630–633). However, it is not clear how transient tolerance is imposed and maintained in the peripheral T cell repertoire during pregnancy.
It is therefore an object of the present invention to identify mechanisms by which rejection of the fetus by its mother are prevented.
It is a further object of the present invention to provide reagents and methods for use thereof for terminating or maintaining pregnancies.